As radio communication becomes more widely accepted, it will be desirable to provide various types of radio communication services to meet consumer demand. For example, support for facsimile, e-mail, video, Internet access, etc., via radio communication systems are envisioned. Moreover it is expected that users may wish to access different types of services at the same time. For example, a videoconference between two users would involve both speech and video support. Some of these different services will require relatively high data-rates compared with speech service that has been conventionally supplied by radio communication systems, while other services will require variable data-rate service. Thus, it is anticipated that future radio communication systems will need to be able to support high data-rate communications as well as variable data-rate communications. Currently 3G wireless systems support such a variety of applications that need different Quality of Services (QoS). The Direct Sequence Code Division Multiple Access (DS-CDMA) based 3G system has greater flexibility in offering variable data-rates, which are particularly important for multimedia and Internet services.
The DS-CDMA system applies pilot-symbol-assisted coherent detection to achieve coherent reverse and forward links. Known pilot symbols are multiplexed with a coded data symbol sequence to make it possible at the receiver to estimate the time varying radio channel gains associated with each resolved multipath component, and then to combine the resolved multi-path components coherently in the Rake receiver.
During communication, the transmitting data-rate may vary every frame. For example, variable-rate voice codec (codec is a generally abbreviated word for COder-DECoder) is used to reduce the average bit rate. The radio link capacity of cellular DS-CDMA mobile radio is limited mainly by the multiple access interference (MAI) from other users transmitting at the same carrier frequency in the same and other cells. The use of discontinuous transmission to match the varying data-rate can reduce the interference to other users. Furthermore, discontinuous transmission is an effective means for prolonging the battery life of mobile terminals. To recover variable-rate data, reliable detection of the data-rate is necessary at the receiver end.
Regardless of the particular technique adopted in a radio communication system for providing variable rate transmission capability, the receiver must know the particular data-rate at which a signal is transmitted in order to properly detect and decode the received signal. Current methods for informing the receiver of the instantaneous data-rate of received signal generally fall into following two categories.
Some systems that explicitly transmit bit rate information (also referred to as Transport Formal Combination Information (TFCI)) along with the data (transmitted signal). Normally, Forward Error Correction (FEC) coding is used on TFCI before transmission. Also, the TFCI is transmitted at a much higher power compared to the user data. On the receiver, a suitable decoder is used so that the TFCI is decoded with sufficient reliability. The TFCI is used in order to inform the receiving side of the currently valid Transport Format Combination, and hence how to decode, de-multiplex and deliver the received data on the appropriate Transport Channels.
Other systems provide the receiver with the capability to “blindly” determine the rate at which data has been transmitted. Such systems do not transmit any TFCI information along with the data. The receiver operates, for example, by decoding for different possible rates and checking for correct Cyclic Redundancy Check (CRC). U.S. Pat. No. 5,566,206 to Butler et al. provides an example of blind rate detection. Generally, blind rate detection schemes are used only when there are fewer possible rates (when there are fewer TFCIs).
Both of the above current techniques have drawbacks. For example, blind rate detection results in relatively complex receivers due to the additional circuitry/logic needed to correctly identify one of a plurality of possible data transmission rates. Also, the blind rate detection scheme assumes that the data-rate changes every frame, which might not be the case, and this can result in increased False Detection Rate (Detection of wrong data-rate), slowing the process of data-rate detection and generally requiring more computation.
The transmission of explicit TFCI also creates design issues. For example, the TFCI is transmitted in the same data frame as the data and they not only include data-rate information but also include spreading factor (SF) information, which is required for decoding data symbols. Therefore, SF is needed for proper decoding of data symbols and the TFCI has to be decoded as fast as possible to avoid storing of the received data. That is, as soon as the data for a frame is available in the transmitter, the TFCI for that frame has to be computed, decoded, and transmitted. Any delay in decoding the TFCI can be undesirable for low-delay services like speech, especially for large frame lengths. When the TFCI is transmitted in the same frame as the data, and if there is any undesirable delay in the decoding, the receiver has to buffer the received signal until it has detected and decoded the TFCI for that frame. Buffering the data in the receiver requires additional cost and complexity. Also, explicit TFCI is transmitted every 10 milliseconds (every frame) and is similarly decoded once every frame on the receiver side. During bad channel conditions, this method of coding and decoding (where it is assumed that data-rate changes every frame) yields poor results as every 10 milliseconds one of many possible explicit TFCIs has to be detected by searching for the maximum output across all encoded bits. Generally, this method requires more computation and can result in slower data-rate detection.
Therefore, there is a need in the art for efficient and reliable data-rate detection to detect at what variable rate a signal is transmitted, in order to properly detect and decode the received signal.